Supported zeolite membranes with controlled microstructures are important in applications as diverse as catalysis, ion-exchange, nuclear waste disposal, light harvesting devices, chemical sensing and gas separations.1-3 Over the past two decades, membranes of various zeolite structure types, formed on inorganic porous supports have shown enormous potential for separations of gas and liquid mixtures.
CO2 separation is one of the most studied applications for zeolite Y membranes because of its industrial significance, such as CO2 capture for carbon sequestration, natural gas purification, and separation of product streams from water gas shift reactions for hydrogen production.7-11 Due to the window size (0.73 nm) of the pores or “supercages” defined by the molecular superstructure of the zeolite Y, there is no steric hindrance for molecules, like CO2, CH4, CO, H2O or N2, to enter in the pores. However, for molecules like CO2, there is preferential interaction with the aluminosilicate framework and the extra framework cations which leads to a favorable combination of sorption and mobility. Thus, with gas mixtures containing CO2, and N2 or CH4, the CO2 permeates preferentially through the zeolite by surface diffusion, while N2 (CH4) is excluded from the faujasite pores due to preferential CO2 adsorption and pore constriction.8,10 
Significant progress has been made in the synthesis of zeolite Y membranes. For example, much prior work has focused on the seeding and secondary growth processes, i.e., processes in which zeolite seed crystals are deposited on a porous substrate followed by growing the seed crystals into a coherent zeolite membrane.17,20 The influence of chemical composition on the particle size of zeolite Y seed crystals has also been studied.21-24 Nonetheless, issues that remain include reproducibility of membrane synthesis, control of defects, and development of membranes highly selective for CO2 separation.